Why do you like/love HeRO?
Overall, I'd say it's because the place is nicer then most other private servers, people are willing to answer questions (sometimes) and even when they don't, they at the very least don't jump to "omg nub" :p
Are the players nice/good to newbies and help them level?
No newbie hate here at all I've seen, helping them level, they'll probably be happy to tell you where to go to level, but partying isn't too big a thing here, although if you hunt around enough, you could probably find one (Depends on class, but in general)
Are the guilds helpful/nice?
Again, yep, WoE can get a little ugly from time to time, but it usually fades out pretty fast.
Are good guilds easy to join?
If you mean good as in powerful, yes, usually just gotta ask, good as in well mannered and nice in general, again, yeah. :p
What is the main class used/has the better advantage?
I see a lot of assassin's, and hunters, alchemists would be a close second, from what I see.
What exactly is with the demon/angel thing? Is there an advantage of being one over the other?
Refer to aaronock's post, they pretty much cover it :p
How hard is it to get custom pets/gear?
Wings can be rough, depending on the set, I'd say the easiest require maybe level 50 - 70, around there. Takes a bit of time though.
Is there anything other than cool custom items that I should know about that is different about HeRO?
Well, there's a small bonus to exp when you're in a party, nothing that's too big a deal, but it certainly helps.
Hope you decide to stick around.